QAYARA AIR BASE, Iraq — The air base that Iraqi forces hope to use as a staging area to take Mosul back from the Islamic State group was almost completely destroyed by the retreating militants, raising new doubts over whether the long-awaited operation will begin this year.

Iraqi forces seized the Qayara air base south of Mosul in July, in what U.S. and Iraqi officials said was a major step toward the eventual liberation of the country's second largest city, which fell to IS in 2014. Prime Minister Haider al-Abadi called on Mosul residents "to get ready for the liberation of their areas."

But Iraqi army commanders stationed at the base say it will take months of reconstruction before it is ready to receive cargo planes and house the tens of thousands of troops needed for the march on Mosul. Their assessments call into question whether Iraq will be able to launch the operation this year, as the prime minister has repeatedly pledged.

"Daesh began destroying this base from the moment they took it over," Col. Karim Rodan Salim said, referring to IS by its Arabic acronym. "No less than 95 percent of the base has been destroyed."

Salim and his men are living in trailers on a patch of tarmac in the shadow of one of the partially collapsed hangers. At a nearby runway dozens of piles of dirt and rocks mark suspected explosives left by IS, Salim said. He estimates it will take at least six months of rebuilding before the base is ready for the 50,000 troops he says will be needed to retake Mosul.

Originally built in 1979, the facility was renamed Saddam Base after it was rehabilitated by the former Iraqi dictator. IS captured the base in the summer of 2014, when it swept across much of northern and western Iraq and drove panicking Iraqi troops out of Mosul. The base is more than six kilometers (four miles) long and has runways large enough to accommodate cargo planes.

The coalition hopes to transform the base into a logistics center ahead of the Mosul operation. The Pentagon announced earlier this month that about 400 soldiers from the 101st Airborne Division will deploy to Iraq as part of that effort. They are among 560 additional troops President Barack Obama approved for the Iraq mission in July.

On Monday, a convoy of dozens of armored coalition vehicles carrying heavy equipment and supplies could be seen along the road leading to the base.

The U.S.-led coalition has stepped up a campaign of airstrikes around Mosul in recent weeks and fast-tracked training programs for Iraqi forces in an effort to meet the prime minister's deadline of launching the operation by year's end.

"The seizure of this base is important because it demonstrates the Iraqi security forces' ability to maintain momentum as Daesh gets weaker and continues to lose territory," said U.S. Army Maj. Gen. Gary Volesky, commander of coalition land forces in Iraq.

IS has retreated from areas around Mosul in recent days, including the nearby town of Qayara, where it set oil wells on fire before the advancing troops. The extremist group has come under mounting pressure on a number of fronts in Syria and Iraq in recent months, losing a string of towns and cities in both countries.

But it has sabotaged infrastructure on its way out, leaving behind a swath of destruction that that in the short-term is hindering further advances by Iraqi forces and in the long-term will require a daunting reconstruction effort.

Across northern and western Iraq, engineering teams are busy rebuilding downed bridges and carefully removing thousands of roadside bombs and other booby-traps. Iraqi forces recaptured Ramadi, the provincial capital of the western Anbar province, six months ago, but the city is still too dangerous for most residents to return.

"What we see here was an organized destruction," Salim said. "But we were expecting it. Daesh never leaves anything behind."